<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>canvas</title>
    <!-- 使用定时器完成下雨特效。 -->
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        canvas {
            vertical-align: middle;
            background: #cc1b1b;
        }
    </style>
</head>

<body>
    <canvas id="myCanvas"></canvas>
    <script>
        // 创建画布
        let myCanvas = document.getElementById('myCanvas')
        let ctx = myCanvas.getContext('2d')
        // 自适应窗口
        let width = myCanvas.width = window.innerWidth
        let height = myCanvas.height = window.innerHeight
        window.addEventListener('resize', () => {
            width = myCanvas.width = window.innerWidth
            height = myCanvas.height = window.innerHeight
        })
        // 绘制雨滴
        let raindropArr = []
        function Raindrop(x, y, w, h, l, r, dr, maxR, a, va) {
            this.x = rand(0, window.innerWidth) // 雨滴的x轴
            this.y = y || 0 // 雨滴的y轴
            this.dy = rand(2, 4) // 雨滴的加速度
            this.w = w || 2 // 雨滴的宽度
            this.h = h || 10 // 雨滴的高度
            this.l = rand(0.8 * height, 0.9 * height) // 雨滴的下落高度
        }
        Raindrop.prototype = {
            draw: function (index) { // 绘制雨滴
                if (this.y > this.l) {
                    ctx.beginPath()
                    ctx.arc(this.x, this.y, this.r, 0, Math.PI * 2)
                    ctx.strokeStyle = `rgba(0,191,255,${this.a})`
                    ctx.stroke()
                } else {
                    ctx.fillStyle = 'skyBlue'
                    ctx.fillRect(this.x, this.y, this.w, this.h)
                }
                this.update(index)
            },
            update: function (index) { // 更新雨滴坐标 运动起来
                if (this.y > this.l) {
                    if (this.a > 0.03) {
                        this.r += this.dr
                        if (this.r > this.maxR) {
                            this.a *= this.va
                        }
                    } else {
                        this.a = 0
                        raindropArr.splice(index, 1)
                    }
                } else {
                    this.y += this.dy
                }
            }
        }
        function rand(min, max) {
            return Math.random() * (max - min) + min
        }
        setInterval(() => {
            let raindrop = new Raindrop()
            raindropArr.push(raindrop)
        }, 100)
        setInterval(() => {
            ctx.fillStyle = 'rgba(0, 0, 0, 0.1)'
            ctx.fillRect(0, 0, myCanvas.width, myCanvas.height)
            for (let i = 0; i < raindropArr.length; i++) {
                raindropArr[i].draw(i)
            }
        }, 30)
    </script>
</body>

</html>
